Signal Processor with side-tone noise reduction for a headset

ABSTRACT

A signal processor ( 105 ) for a headset ( 101 ) configured with a microphone terminal ( 106 ) for receiving a microphone signal, a loudspeaker terminal ( 107 ) for outputting a loudspeaker signal, and a far-end terminal ( 108 ) for communicating an inbound signal and an outbound signal with a far-end; comprising: a side-tone path ( 110 ) configured to generate a side-tone signal from the microphone signal via a controllable side-tone filter; wherein a side-tone filter controller ( 114 ) receives the microphone signal and computes a first noise estimate with a signal-to-noise level of the microphone signal at respective frequency bands and based thereon controls the side-tone filter ( 111 ) to improve or optimize a signal-to-noise ratio.

INTRODUCTION/BACKGROUND

A headset for voice communications (e.g., a Bluetooth™ headset, a DECTheadset or a wired headset, such as a USB headset) typically contains aloudspeaker for reproducing a far-end audio signal at one of the user'sears and a voice microphone for receiving the headset wearer's speech.

The loudspeaker is positioned at one or both of the headset wearer'sears, and the voice microphone is arranged within the headset to bepositioned during use to receive the user's speech with an acceptablyhigh signal-to-noise ratio. The voice microphone is typically located,for example, within a headset housing, an ear-cup, in a headset boom orother protrusion that extends from such a housing toward the user'smouth, or on a cord that carries audio signals to and from the cellulartelephone.

A headset may attenuate sound from the environment around the headsetwearer by an ear-cup or an active noise-cancelling technique such thatthe headset wearer is protected from disturbing sounds around him/her. Aheadset may also attenuate sound from the environment around the headsetwearer before transmission to a far end e.g. in case the voicemicrophone is a directional microphone e.g. implemented usingbeamforming.

It may happen that a headset attenuates not only sound from theenvironment but also the headset wearer's own speech. This causesproblems for the headset wearer to adapt his speech and in particularhow loud he/she talks since he/she cannot hear him-/herself.

To overcome that problem, the headset may be configured with a feedbackpath whereby a small amount of the headset wearer's own speech (pickedup by the voice microphone) is fed back to the headset wearer's ear viathe loudspeaker. This is also known as a side-tone. The side-tone isoften attenuated or amplified to reflect the amount of feedback a personwill perceive when speaking in a room without wearing a headset.

The further away from the headset wearer's mouth the voice microphone issituated, the more sound from the surroundings relative to speech fromthe headset wearer will be picked up and reproduced by the loudspeakerin his/hers ears. This may be a problem since it is often preferred todispense with microphone booms or similar extensions.

It should be noted that conventionally sometimes a loudspeaker in aheadset is denoted a receiver.

RELATED PRIOR ART

U.S. Pat. No. 7,330,739 describes a wireless communication device thatprovides a side-tone to the device user. The device converts an outboundanalogue audio signal from a microphone to an outbound audio bit-streamfrom which a side-tone bit-stream is extracted. The side-tone bit-streamis extracted between a sigma-delta ADC and before a FIFO buffer. Thedevice also converts an inbound digital audio signal, such as a 13 bitssignal, to an inbound audio bit-stream, such as a 1 bit signal, by meansof a sigma-delta converter. The side-tone bit-stream is added to theinbound audio bit-stream after a FIFO buffer and before a low-passfilter, such as a bi-quadratic switch capacitor filter, which convertsthe audio bit-stream from the sigma-delta converter to an analoguesignal. It is claimed that problems in connection with digitalimplementations related to latency of the side-tone are overcome. It isalso claimed that since both the inbound audio bit-stream and theside-tone bit-stream are filtered by the same bi-quadratic switchcapacitor filter the ‘richness’ of the two audio signals issubstantially the same and therefore they sound equally ‘rich’ or‘loud’.

U.S. Pat. No. 9,053,697 describes a headset with an active noisecancellation (ANC) system. The ANC system is coupled to a noisereference microphone that, contrary to a voice microphone, is directedat an ear canal of the headset wearer during use of the headset to pickup an external noise reference signal, generate an anti-noise waveformfrom the noise reference signal, and reproduce the anti-noise waveformthrough one or more loudspeakers in the headset. The use of an ANCtechnique may reduce the level of background noise that reaches theheadset wearers ear (by up to twenty decibels) while delivering usefulsound signals, such as music and far-end voices received from a far end.

It is described that a mixer may be configured to mix some audibleamount of the headset wearer's speech (a near-end speech estimate) intoan audio signal to be reproduced by the one or more loudspeakers in theheadset; this is also known as a side-tone. The near-end speech estimatemay be any among: a near-end signal (which is based on information froma voice microphone), a noise-suppressed signal from a noise suppressionfilter and other signals. In one example, the noise suppression filteris configured to distinguish speech frames of its input signal fromnoise frames of its input signal and to produce noise-suppressed signalto include only the speech frames. The noise suppression filter isalternatively implemented as spatially selective processing filter.

The Internet article “Adding Sidetone to Skype” (available athttps://channel9.msdn.com/coding4fun/articles/Adding-Sidetone-to-Skype)describes a software plug-in for Skype™ that adds a side-tone to theearpiece of a headset. In one example, the side-tone is added via adigital low-pass filter of the Butterworth type implemented as an IIRfilter.

However, it remains a problem that noise from the surroundings may bepicked up by the voice microphone and end up in the headset wearer's earvia the side-tone and not least in the far-end.

SUMMARY

There is provided a signal processor for a headset configured with amicrophone terminal for receiving a microphone signal, a loudspeakerterminal for outputting a loudspeaker signal, and a far-end terminal(108) for communicating an inbound signal and an outbound signal with afar-end; comprising:

-   -   an outbound path from the microphone terminal to the far-end        terminal;    -   a side-tone path configured to generate a side-tone signal from        the microphone signal via a side-tone filter;    -   an inbound path from the far-end terminal to the loudspeaker        terminal and comprising a combiner configured to combine the        side-tone signal and the inbound signal to generate the        loudspeaker signal; and    -   a side-tone filter controller that receives the microphone        signal and computes a first noise estimate with a        signal-to-noise level of the microphone signal at respective        frequency bands and based thereon controls the side-tone filter        to improve a signal-to-noise ratio of the side-tone signal;        wherein the side-tone filter is a controllable side-tone filter.

Thereby a side-tone signal may be generated wherein noise picked up by avoice microphone of the headset, while the headset wearer speaks, may beattenuated. Since the side-tone filter is controlled based on the firstnoise estimate with a signal-to-noise level of the microphone signal atrespective frequency bands the side-tone filter may attenuate frequencybands with a poor signal-to-noise ratio relatively to frequency bandswith a better signal-to-noise ratio. The amount of attenuation may bedetermined based on the signal-to-noise ratio at respective frequencybands. This greatly improves the acoustic impression experienced by theheadset wearer since a side-tone with significantly less noise isinduced into the headset wearer's ears. The noise estimate may be usedto describe at which frequencies the noise predominantly occurs and atwhat level the noise occurs.

The side-tone filter may be controlled to improve a signal-to-noiseratio of the side-tone signal by attenuating the microphone signal in afrequency band wherein the first noise estimate is indicative of arelatively low signal-to-noise ratio relative to a frequency bandwherein the first noise estimate is indicative of a relatively highsignal-to-noise ratio.

The signal-to-noise level of the side-tone signal may be evaluated overmultiple frequency bands, such as over all or selected ones of therespective frequency bands or over an audio band, such as over an audioband of interest.

The side-tone filter may be an Infinite Impulse Response, IIR, filtersuch as a digital IIR filter with discrete filter coefficients. IIRfilters have a lower latency than FIR filters and are advantageous sincethey introduce a lower latency. Latency in generation of the side-tonemay be very disturbing for the headset wearer; especially latency longerthan about 5 milliseconds is considered to be disturbing.

The side-tone filter implements a transfer function with poles and/orzeroes that are controllably arranged to exhibit a desired gain and/orphase characteristic over an audio band of interest. The desired gaincharacteristic may comprise one or more of a low-pass band, a high-passband, a stop-band, a pass-band, a “low shelf”, and a “high shelf”. A“low shelf” has a gain above or below 0 dB below a cut-off frequency anda gain of 0 dB above. In contrast, a “high shelf” has a gain of 0 dBbelow the cut-off frequency and a gain above or below 0 dB above thecut-off frequency. In some aspects a “low shelf” attenuates signalsbelow the cut-off frequency and a “high shelf” attenuates signals abovethe cut-off frequency. A pass band or stop band may be implemented by apeak-filter.

The audio band of interest may be from 20 Hz to 20 KHz or a wider ornarrower audio band. In general a cut-off frequency is a frequency atwhich the gain has fallen to a −3 dB level relative to a nearest 0 dBlevel at a higher or lower frequency.

In some aspects the signal-to-noise estimator sits in a control path,extending from the microphone terminal to the side-tone filter, with acontroller that controls the controllable side-tone filter. Thus, thecontrol path receives the microphone signal.

The microphone signal is filtered by the controllable side-tone filterat a filter rate, typically, at a regular sample rate or a decimatedsample rate. The control path controls the controllable side-tone filterat a control rate, which may be the same as the filter rate or a lowerregular or irregular rate. The controllable side-tone filter is adaptedat the control rate.

The signal-to-noise estimator may estimate the signal-to-noise level ofthe microphone signal at respective frequency bands related to“frequency bins”. The term “frequency bin” often relates to a digital(discrete) implementation of the signal-to-noise estimator wherein adiscrete range of frequencies and discrete resolution of an estimatedspectrum depends on sampling rate and the number of sampling points. Thespectrum is computed for a number of points—often denoted “bins” or“frequency bins”. However, it should be emphasized that an analogueimplementation is foreseeable too.

In some embodiments the side-tone filter, in the side-tone path,comprises multiple filter stages that are individually controlled basedon the first noise estimate.

Thereby it is possible to realize a low latency and flexible filterstructure that can be controlled in terms of frequency and gain toattenuate noise in selected frequency bands. Each of the individuallycontrollable filter stages may be an IIR filter e.g. of the bi-quadtype.

The multiple filter stages may implement similar or different filterstructures and similar or different filters located at overlapping ornon-overlapping frequency bands. A filter may be considered to notoverlap another filter in case one of the filters alters the gain byless than a threshold gain such as less than 3 dB or less than 1 dB asexamples.

Each of the individually controllable filter stages may be controlled bysetting filter coefficients in a predetermined filter structure e.g. abi-quad direct form or bi-quad direct form 2, or a bi-quad transposedform. The filter stages may be controlled via parameters such as acentre frequency, bandwidth and gain for a peak filter, via a cut-offfrequency, steepness and a gain level for a shelf filter, and via acut-off frequency and steepness for a low-pass filter; however otherparameters may be used additionally or alternatively.

In some embodiments the multiple filter stages are configured from agroup consisting of: multiple peak filters and a high-shelf filter.

Thereby the multiple peak filters may be arranged to specificallyattenuate signals in accordance with the signal-to-noise ratio atfrequencies where there is a poor signal-to-noise ratio.

The peak filters may be configured in terms of a centre frequency, again and a bandwidth. The gain of the peak filter may be controlled atgains below 0 dB. The high-shelf filter may be configured in terms of ashelf midpoint frequency. The gain of the high-shelf filter at the shelfmay be controlled at gains below 0 dB.

In some aspects the group consists of less than six peak filters, e.g.between three and six peak filters, and one or two high-shelf filters.Thereby a good trade-off between low complexity, low latency andsignal-to-noise ratio improvement is achieved. In some aspect a seriesof five peak filters and one high-shelf filter, all of the bi-quadratictype, has been found by experiments to give a good trade-off.

In some aspects the filter stages are arranged in series, wherein ahigh-shelf filter is the last filter in the series. Thereby thehigh-shelf filter, which follows the peak filters, may soften undesiredeffects of filtering the microphone signal by multiple peak filters. Thehigh-shelf filter may be controlled to have an appropriate shelfmid-point frequency and attenuation at the shelf to achieve thispurpose. An advantage of a high-shelf filter, that attenuates higherfrequencies, is that it is possible to attenuate noise while onlydegrading intelligibility of speech marginally.

In some embodiments the side-tone filter controller executes aniterative process of:

-   -   determining parameters for a filter stage based on the first        noise estimate;    -   computing a frequency domain gain estimate of the frequency        domain gain transfer function for a filter stage;    -   computing a second noise estimate by subtracting the frequency        domain gain estimate from the first noise estimate;    -   updating the first noise estimate to be in accordance with the        second noise estimate;        wherein the filter stages are controlled via coefficients        computed from the parameters for a respective filter stage        computed over iterations of the iterative process.

An advantage of the iterative estimation process, when the filter stagesare of the IIR type, is that the side-tone filter configured thereby hasvery low latency while still providing a good suppression of noise inthe side tone signal.

It has been realized that the computational effort in computing theeffect on the noise estimate of applying the selected filter, whichwould be to filter a portion of the microphone signal with the selectedfilter, may be too big for the signal processor. However, by iterativelycomputing an estimate of the effect of applying the selected filter, thecomputational effort may be much reduced.

By such iterative determining of parameters for filter stages in theside-tone filter, based on a noise estimate which is updated from oneiteration to another, wherein a frequency domain gain transfer functionfor a filter stage is estimated—instead of being completely computed—theside-tone filter can be more frequently and more smoothly updated tokeep up with the optimization of signal-to-noise ratios at respectivefrequency bands.

The iterative process starts out, in its first iteration, from the firstnoise estimate computed from the microphone signal; whereas in a secondand further iteration, the first noise estimate is based on estimatingthe effect of filtering the microphone signal by a filter for whichparameters are determined in a succeeding iteration.

The frequency domain gain estimate may be supplemented by a frequencydomain phase estimate. An advantage of computing a frequency domainestimate for a filter stage—instead of applying the filter—is that theupdated noise estimate (the second noise estimate) can be computedbin-by-bin without time-consuming convolution computations.

The frequency domain gain estimate may be approximated by a geometricapproximation such as a corner frequency approximation.

In some aspects coefficients for a filter stage are implemented in afilter stage during the course of the iterative process. In otheraspects, the filter stages are controlled via the coefficients when theiterative process has completed an iteration round. In the latter case,the signal processor keeps a representation, over the course of theiterative process, of each filter model computed at respectiveiterations of the process.

In some embodiments determining parameters for a filter stage comprisesfitting a filter that has a largest possible gain-bandwidth productwhile optimizing the signal-to-noise ratio in an audio band of interest.

Thereby it is ensured that the filter stages that are available areutilized most effectively in terms of improving the signal-to-noiseratio as far as possible given the available filter stages. This in turnimproves the acoustic quality perceived by the headset wearer.

The fitting of such a filter, with a largest possible gain-bandwidthproduct, may be performed using a numerical optimization process.

It should be noted that “a filter” in connection with “fitting a filter”is a representation of a filter either by parameters for type of filtersuch as centre frequency and gain for a peak filter of the bi-quad typeor by coefficients for an implementation form or structure.

In some embodiments fitting a filter comprises:

-   -   over a range of frequencies, iteratively selecting a frequency;        and    -   determining a gain-bandwidth product of a band centred at the        frequency and determining a gain factor thereof based on the        signal-to-noise ratio and determining a bandwidth factor thereof        as the bandwidth wherein the signal-to-noise ratio is not better        than approximately half the signal-to-noise ratio at the        frequency;    -   when the range of frequencies has been iterated, selecting a        favourable gain-bandwidth product, such as the largest        gain-bandwidth product, and estimating parameters for a        band-attenuate filter, such as a peak-filter, with a centre        frequency and a gain to approximately fit the determined band.

Thereby the signal processor can perform a fast and robust filterfitting process. The filter fitting process has shown to be robustdespite of the fitting process being based on a much simplifiedrepresentation of a filter in its selection of a “best fitting” filter.

In some embodiments fitting a filter comprises:

-   -   fitting a high-shelf filter by selecting a predetermined shelf        mid-point frequency and determining a shelf gain, such as an        attenuation, from an average of the signal-to-noise ratio at        frequencies above the shelf mid-point frequency.

Such a high-shelf filter has shown to significantly reduce distortion orsoften audible artefacts introduced by regularly or irregularly changingthe characteristics of the filter stages in the side-tone filter. Thequality of the acoustics obtained by introducing a side-tone is therebyimproved.

The high-shelf filter further enables suppression of high-frequencynoise in a single filter with an effect comparable to that of multiplepeak or band filters, without significant degradation of speech quality,and with significantly less latency than that achievable with suchmultiple filters.

In some embodiments the signal processor:

-   -   at a first time, computes a reference gain based on a minimum        attenuation in the noise estimate and scales the microphone        signal based on the reference gain; and    -   at a second time, following the first time, performs the        iterative process set out above.

Thereby the efficacy of the noise reduction is enhanced at least incases wherein very little speech is detected. The reference gain may beapplied uniformly across frequency bins.

The reference gain may be computed from the noise estimate by searchingfor a minimum attenuation, this value is applied as a reference gain tothe full bandwidth. Applying the reference gain to compensate the noiseestimate improves the filter fitting process e.g. when very littlespeech is detected.

In some embodiments the signal processor:

-   -   at a second time, computes a frequency domain gain estimate of a        high-shelf filter, and then computes a second noise estimate by        subtracting the frequency domain gain estimate from the first        noise estimate;    -   at a third time, following the second time, performs the        iterative process set out above, wherein the filter is a        stop-band filter or a peak-filter.

In some aspects, during configuration of the filter stages, the secondtime follows the first time; thus, firstly a uniform gain is estimatedand applied for the following estimation purposes, secondly thehigh-shelf filter is estimated and thirdly the stop-band filter orpeak-filter is estimated.

It should be noted that some aspects processing of the microphone signal(in real time) comprises firstly scaling the microphone signal by theestimated uniform gain, secondly filtering the scaled microphone signalby filter stages comprising the estimated band-attenuate of peak filtersand thirdly applying a last filter stage configured as a high-shelffilter.

In some aspects, the order of computing parameters and/or coefficientsfor the filter stages, e.g. the peak and/or band filters, is the same asthe filter stages' order in the series of filter stages in thecontrollable side-tone filter.

In some aspects, computing parameters and/or coefficients forpeak-filter or band filter stages follows computing of one or both ofoverall gain and parameters and/or coefficients for the high-shelffilter. Thus, gain and parameters and/or coefficients for the high-shelffilter are computed first and then coefficients for peak-filter or bandfilter stages are computed. It should be noted that the order in whichparameters are computed for the filter stages and gain stages plays arole since the noise estimate is updated in between and is used for asubsequent computation.

In some embodiments a cost function is defined for an optimizationprocess; wherein the cost function is a frequency domain estimate of thesignal-to-noise ratio which in a first iteration is computed from themicrophone signal and in subsequent iterations is computed from the costfunction and estimates in the frequency domain of selected filters; andwherein the optimization process iteratively selects a filter with anoptimization strategy to optimize the cost function and then updates thecost function.

In some embodiments determining parameters for a filter stage by fittinga filter comprises dampening the rate and/or distance the centrefrequency and/or bandwidth is allowed to change over time.

Thereby undesired effects, such as introduction of distortion oracoustic artefacts, caused by regularly or irregularly changing thefilter characteristics may be reduced and made less audible or morepleasant to the human ear. The high-shelf filter may be controlled tohave an appropriate shelf mid-point frequency and attenuation at theshelf to enhance suppression of such undesired effects.

The rate and/or distance the centre frequency and/or bandwidth isallowed to change over time may be controlled by a low-pass filter typecomprising a low-shelf filter or a controller.

Several restrictions may be applied to suppress audible artefactsresulting from rapid and/or large shifts in bi-quad filter coefficients:

The filter fitting process may be restricted to, for a selected filter,search within a respective predefined frequency range to limit thefilter's shift in centre or cut-off frequency from one iteration to thenext. This ensures that peak filter stays within the respectivepredefined frequency range.

The filter fitting process may be further restricted such that a filterdoes not deviate more than a certain percentage or amount from thefilter computed in a previous iteration. So, the filter fitting processmay be allowed to search only in a frequency range about the filter'sprevious centre or cut-off frequency and within the respectivepredefined frequency range.

In some embodiments the signal processor comprises a voice activitydetector which indicates presence of speech; wherein a first amplitudeor power spectrum for the microphone signal is computed at a time whenthe voice activity detector is detecting speech; wherein a secondamplitude or power spectrum for the microphone signal is computed at atime when the voice activity detector is not detecting speech; andwherein the signal-to-noise ratio is computed from the first amplitudeor power spectrum and the second amplitude or power spectrum.

There is also provided a signal processing method for generating aside-tone in a headset, comprising:

-   -   generating a side-tone signal via a side-tone filter and from a        microphone signal picked up via a microphone of the headset,    -   combining the side-tone signal and an inbound signal to the        headset to generate a loudspeaker signal for a loudspeaker of        the headset; and    -   computing a first noise estimate of the microphone signal at        respective frequency bands and based thereon controlling the        side-tone filter to improve a signal-to-noise ratio of the        side-tone signal;        wherein the side-tone filter is a controllable side-tone filter.

Additional, optional, aspects or steps of the signal processing methodare set out in connection with the above signal processor.

There is also provided a computer-readable medium carrying a programconfigured to perform the signal processing method, wherein the signalprocessing method is a computer-implemented method run on a computer.There is also provided a headset comprising a voice microphone, anearpiece with a loudspeaker and a signal processor according to any ofthe preceding claims.

The headset may be of the type having a microphone boom ( ) hosting themicrophone, or of the type wherein the microphone is hosted in aninterposed housing on a wire connection of the headset or of the typewherein the voice microphone is hosted with the loudspeaker in a commonhousing, such as an earpiece, earbud or ear cup.

Here and in the following, the terms ‘signal processor’, ‘filter’,‘controllable filter’, ‘side-tone filter’, ‘controllable side-tonefilter’ ‘controller’ etc. are intended to comprise any circuit and/ordevice suitably adapted to perform the functions described herein. Inparticular, the above term comprises general purpose or proprietaryprogrammable microprocessors, Digital Signal Processors (DSP),Application Specific Integrated Circuits (ASIC), Programmable LogicArrays (PLA), Field Programmable Gate Arrays (FPGA), special purposeelectronic circuits, etc., or a combination thereof.

BRIEF DESCRIPTION OF THE FIGURES

A more detailed description follows below with reference to the drawing,in which:

FIG. 1 shows a block diagram of a headset with a signal processor;

FIG. 2 shows a side-tone path with a controllable side-tone filtercomprising multiple filter stages;

FIGS. 3 through 7 illustrates the operation of a side-tone filtercontroller during configuration of side-tone filter stages;

FIG. 8 shows a more detailed block diagram of a side-tone filter and aside-tone filter controller; and

FIG. 9 shows a flowchart of controlling a side-tone filter.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a headset with a signal processor. Theheadset 101 is depicted in a 3D representation and in a diagrammaticrepresentation.

As shown in the 3D representation, the headset 101 may have a housing103 with an ear-cup, of the on-the-ear type or over-the-ear type and amicrophone boom 104 extending from the housing 103 and having amicrophone end or microphone compartment 102 hosting one or moremicrophones, e.g. a directional microphone, for picking up a headsetwearer's speech. A headband or head support is provided for holding theheadset on the headset wearer's head. In some embodiments, the headsetmay have an additional ear-cup for the other ear. In some embodimentsthe ear-cups are of the earbud type and the microphone boom 104 isreplaced by an in-line microphone which is attached to a cord. The cordmay connect to the headset to a computer 117, a desk telephone 116, or asmartphone 115—in some embodiments via a base-station for the headset(not shown). In some embodiments the headset is a wireless headsetcommunicating wirelessly with one or more of the computer 117, the desktelephone 116, the smartphone 115 or the base station.

As shown in the diagrammatic representation, the headset 101(represented by a dashed-line box) comprises a loudspeaker 119 and amicrophone 120. Further circuitry such as a preamplifier and ananalogue-to-digital converter for the microphone and such as anamplifier for driving the loudspeaker is not shown.

The headset 101 has a signal processor 105, which may be accommodated inthe housing 103. The signal processor 105 is configured with amicrophone terminal 106 for receiving a microphone signal from themicrophone 120, a loudspeaker terminal 107 for outputting a loudspeakersignal to the loudspeaker 119, and a far-end terminal 108 forcommunicating an inbound signal and an outbound signal with a far-end.The inbound signal and the outbound signal may be communicated on acommon terminal as a bi-directional digital signal or on respectiveinbound and outbound terminals.

Here and in the following, a far-end refers to a communications device,audio receiver or system to which the headset wearer's speech, asreproduced by the microphone 120 and an outbound path of the headset, istransmitted as an outbound signal and/or a communications device, audiosource or system from which an audio signal is received as an inboundsignal and reproduced in the loudspeaker towards the headset wearer'sear. An inbound signal and an outbound signal refer to any type of audiosignal received from and transmitted to the far end, respectively.

An outbound path 109 extends from the microphone terminal 106 to thefar-end terminal 108. The outbound path 109 may comprise circuitry, suchas an analogue-to-digital converter, a filter, and/or a buffer, forprocessing and/or communicating the microphone signal to the far-endterminal.

An inbound path 112 extends from the far-end terminal 108 to theloudspeaker terminal 107 and comprises a combiner 113 configured tocombine a side-tone signal and the inbound signal to generate theloudspeaker signal. The combiner may be configured as a digital adder oran analogue adder as the case may be. In this audio context the combineris sometimes denoted a mixer or an audio mixer. The inbound path 112 maycomprises signal processing components or circuitry for one or more ofnoise suppression, automatic level (gain) adjustment, and transientprotection.

A side-tone path 110 extends from the microphone terminal 106 or fromthe outbound path 109 and is configured to generate a side-tone signalfrom the microphone signal via a controllable side-tone filter 111. Thecontrollable side-tone filter 111 is dynamically controlled to minimizeor reduce the amount of noise picked up by the microphone 120 andreproduced in the side-tone. The controllable side-tone filter 111 iscontrolled by a side-tone filter controller 114 that receives themicrophone signal and computes a first noise estimate. The first noiseestimate is computed based on the microphone signal and comprises asignal-to-noise level at respective frequency bands. Based thereon theside-tone filter controller 114 controls the side-tone filter 111 tooptimize or improve a signal-to-noise ratio at the respective frequencybands.

In this respect, noise may be represented by the microphone signal attimes when speech (the headset wearer's voice) is not detected. For thispurpose, a so-called voice activity detector, VAD, may indicate whenspeech is present and when speech is not present. In some aspectsindication of when speech is present and when speech is not present isgenerated per frequency bin or frequency band.

One example of a voice activity detection operation includes comparinghigh-band and low-band energies of the signal to respective thresholdsas described, for example, in section 4.7 (pp. 4-49 to 4-57) of the3GPP2 document C.S0014-C, v1.0, entitled “Enhanced Variable Rate Codec,Speech Service Options 3, 68, and 70 for Wideband Spread SpectrumDigital Systems,” January 2007 (available online at www.3gpp.org).

The controllable side-tone filter 111 may improve the signal-to-noiseratio when the noise estimate is indicative of noise that is differentfrom e.g. white noise. However, since both noise and speech typicallyhave a band-limited character, e.g. at multiple bands, controlling thecontrollable side-tone filter to attenuate microphone signals at suchone or more bands may improve the signal-to-noise ratio considerably.This will be explained in more detail in the below.

It should also be noted that the signal processor 105 may comprise acommunications module 121 which is connected to the far-end terminal 108and configured to communicate the inbound and outbound signal with oneor more of a smart-phone 115, a desk telephone 116, a personal computer117, a base station (not shown) and other types of devices. Thecommunications module 121 may be configured to communicate via wired orwireless connections e.g. via one or more of USB, DECT, and Bluetooth™.

FIG. 2 shows a side-tone path with a controllable side-tone filtercomprising multiple filter stages. In an embodiment, the controllableside-tone filter 111 comprises a series of signal processing stagescomprising a gain stage 201 and multiple filter stages 202 through 206which are individually controllable via the side-tone filter controller114.

The series of the gain stage 201 and the multiple filter stages 202through 206 comprises a first peak filter 202, a second peak filter 203,a third peak filter 204 and a fourth peak filter 205 followed by ahigh-shelf filter 206. The gain stage 201 is the first signal processingstage, followed by the peak filters 202 through 205, followed by thehigh-shelf filter 206.

The signal processing stages (201 through 206) are configured for lowlatency to avoid that the headset wearer hears his own voice with adelay. The filter stages 202 through 206 may be implemented as IIRfilters e.g. as bi-quad filters. The bi-quad filters may be implementedin various ways e.g. as a transposed direct form 1 or a transposeddirect form 2.

The signal processing stages comprising the filter stages and the gainstage are controlled via the side-tone filter controller 114 which setsfilter coefficients and/or gain coefficients to obtain a desiredtransfer function for the controllable side-tone filter 111.

It should be noted that the signal processing performed by the signalprocessing stages of the side-tone filter is performed in the orderdescribed above, however, the signal processing stages may be configuredby the side-tone filter controller 114 in an order differenttherefrom—for instance a coefficient of the gain stage 201 may be setfirstly, then the coefficients of the high-shelf filter 206 and finallythe coefficient of the peak-filter stages 202 through 205.

Other types of low-latency filter stages and combinations thereof areforeseeable.

The side-tone filter controller will be described in greater detailbelow with reference to approximated frequency gain responses and noiseestimates and with reference to a more detailed block diagram andflowchart.

FIGS. 3 through 7 illustrates the operation of the side-tone filtercontroller 114 during configuration of side-tone filter stages. Thefigures comprises each two diagrams whereof an uppermost diagram shows anoise estimate for the microphone signal (when not yet filtered or whenfiltered by one or more filter stages) and an estimated or approximatedfrequency domain gain characteristic for a filter stage and a lowermostdiagram shows an updated noise estimate for the microphone signal whenfiltered by the one or more filter stages.

The diagrams are Cartesian coordinate systems with their abscissa axisrepresenting discrete frequencies or narrow frequency bands (sometimesdenoted bins) which may correspond to the number of points (or afraction thereof) used in a time-domain to frequency-domaintransformation such as the FFT; and their ordinate axis representinggain, amplitude or power represented on a logarithmic scale (indecibels, dB). The ordinate axis extends from −20 dB (lowermost) to 0 dB(uppermost). Thus, a higher noise level (poorer signal-to-noise ratio)is represented by a curve or portion of the curve being located at alower position in the diagram.

The side-tone filter controller 114 configures the filter stagesone-by-one as illustrated in connection with the flowchart describedbelow and in an order which may or may not deviate from the order inwhich the filter stages are arranged.

FIG. 3, uppermost diagram, shows a noise estimate 302 for the microphonesignal and a frequency domain gain estimate 301 (approximated) of afrequency domain gain transfer function for the high-shelf filter 206.The high-shelf filter may be configured (also denoted fitted) by apredefined mid-shelf frequency and a shelf gain which is based on anaverage noise level of the microphone signal at or above the mid-shelffrequency.

The lowermost diagram shows an updated noise estimate 303 (approximated)following an approximated filtering by the high-shelf filter 206.

The approximated filtering can be performed, by subtraction operationson gain values in decibels from the gain estimate 301 for the high-shelffilter 206 and noise levels or signal-to-noise ratios in decibels fromthe noise estimate. The approximated filtering deviates from acorresponding ‘real’ or more accurate simulated filtering, but is muchfaster to compute. Thereby, the side-tone filter can be adapted to apresent noise situation much faster.

FIG. 4, uppermost diagram, shows the noise estimate 303 for themicrophone signal following approximated filtering by a high-shelffilter as depicted in connection with FIG. 3 and a frequency domain gainestimate 401 (approximated) of a frequency domain gain transfer functionfor the peak filter 202.

The peak filter 202 may be fitted by a filter fitting process of: over arange of frequencies, iteratively selecting a frequency and for eachfrequency:

-   -   determining a gain-bandwidth product of a band centred at the        frequency and determining a gain factor thereof based on the        signal-to-noise ratio and determining a bandwidth factor thereof        as the bandwidth wherein the signal-to-noise ratio is not better        than approximately half the signal-to-noise ratio at the        frequency; and    -   when the range of frequencies has been iterated, selecting a        favourable gain-bandwidth product, such as the largest        gain-bandwidth product, which is illustrated by the dashed-line        box 403, and estimating parameters for the peak-filter, with a        centre frequency (at the centre of the dashed-line box 403) and        a gain (corresponding to the height of the dashed-line box 403)        to approximately fit the determined band.

The filter fitting process results in an approximated frequency domaingain estimate 401 (approximated) of a frequency domain gain transferfunction for the peak filter 202.

In some embodiments the range of frequencies which the iterations runover, are limited by different bands e.g. limited by the band designated“1” for fitting the peak filter 202. The other peak filters may befitted using a limited range of frequencies which the iterations runover such as indicated by the bands designated “2”, “3”, “4”, and “5”.In some embodiments the bands have overlapping frequencies.

The lowermost diagram shows an updated noise estimate 402 (approximated)following an approximated filtering by the peak filter 202. Theapproximated filtering can be performed by subtraction operations asmentioned above.

FIG. 5, uppermost diagram, shows the noise estimate 402 for themicrophone signal following approximated filtering by the high-shelffilter and peak filter as depicted in connection with FIGS. 3 and 4, anda frequency domain gain estimate 501 (approximated) of a frequencydomain gain transfer function for the peak filter 203. The peak filter203 may be fitted by the filter fitting process described above.

As described above, in connection with the filter fitting process afavourable gain-bandwidth product, such as the largest gain-bandwidthproduct, is illustrated by the dashed-line box 503; a centre frequencyis located at the centre of the dashed-line box 503 and a gaincorresponds to the height of the dashed-line box 503. This may serve asa representation of an approximate filter fit.

The lowermost diagram shows an updated noise estimate 502 (approximated)following approximated filtering by the high-shelf filter 206, the peakfilter 202 and the peak filter 203.

FIG. 6 shows in a similar manner as described above, the noise estimate502 following approximated filtering by the high-shelf filter 206, thepeak filter 202 and the peak filter 203 and a frequency domain gainestimate 601 (approximated) of a frequency domain gain transfer functionfor the peak filter 204. A favourable gain-bandwidth product, such asthe largest gain-bandwidth product, is illustrated by the dashed-linebox 603; a centre frequency is located at the centre of the dashed-linebox 603 and a gain corresponds to the height of the dashed-line box 603.

A resulting updated noise estimate 602 is shown lowermost.

FIG. 7 shows in a similar manner as described above, the noise estimate602 following approximated filtering by the high-shelf filter 206, thepeak filter 202, the peak filter 203 and the peak filter 204 and afrequency domain gain estimate 701 (approximated) of a frequency domaingain transfer function for the peak filter 205. A favourablegain-bandwidth product, such as the largest gain-bandwidth product, isillustrated by the dashed-line box 703; a centre frequency is located atthe centre of the dashed-line box 603 and a gain corresponds to theheight of the dashed-line box 703.

A resulting updated noise estimate 702 is shown lowermost.

Albeit in an approximated way, the resulting updated noise estimate 702compared to the estimated noise estimate 302 is indicative of the noisesuppression achieved by the controllable side-tone filter. When theside-tone filter stages are configured with coefficients, computed fromthe parameters estimated as illustrated above, it may be experiencedthat achieved noise suppression is better than indicated by theapproximation.

FIG. 8 shows a more detailed block diagram of a side-tone filter and aside-tone filter controller.

The side-tone filter controller 114 comprises an FFT component 802 thatreceives the microphone signal and computes an FFT frequency amplitudeor power spectrum from a sampled time-domain microphone signal. Based onthe FFT frequency amplitude or power spectrum a noise estimator 803computes a noise estimate as illustrated above for the microphonesignal. The noise estimator 803 may base its computation of the noiseestimate on a signal, from a voice-activity-detector 804, indicative ofwhether speech is present in the microphone signal or not.

Based on the noise estimate an optimal filter search, OFS, component 805performs a search for parameters for an optimal filter as describedabove and passes the parameters found onto a filter parameter to filtercoefficients, FP2FC, component 806 which computes filter coefficientsand configures the filter stages 202 through 206 accordingly. Theparameters are also passed on to an approximator, APX, 807 whichcomputes an approximated frequency domain gain response based on thefilter parameters, whereby noise estimator 803 can compute an updatednoise estimate.

Based on the noise estimate, a max gain component 808 computes a gaincoefficient for the gain stage 201.

As shown the signal processor may comprise a beam former 801 whichgenerates the microphone signal from two or more microphones. In someembodiments the beam former is omitted.

FIG. 9 shows a flowchart for a process of controlling a side-tonefilter. The flowchart is generally designated by reference numeral 900.The process of controlling the side-tone filter may be performed by theside-tone filter controller 114 or the signal processor 105. Controllingof the side-tone filter is performed as a repeated process which may beinitiated on an interrupt or as a continuous process. Controlling startsin step 901 by computing a noise estimate based on the microphonesignal. From the noise estimate an optimal gain is computed in step 902;the optimal gain is used in step 912, which sets a gain coefficient tothe gain stage 201, and in step 903, which updates the noise estimatebased on the computed gain. Step 912 computes filter coefficients fromfilter parameters.

Following step 903, a high-shelf filter is fitted in step 904,comprising determining parameters for the high-shelf filter, based onwhich an approximation of the high-shelf filter is computed in step 905.The noise estimate is then updated in step 906 to take into account theeffect of the high-shelf filter, as described above. The parameters forthe high-shelf filter computed in step 904 are also used in step 912,which sets filter coefficients for the high-shelf filter stage 206.

Following step 906, one or more peak filters are fitted in step 907using a similar approach, wherein an approximation of a peak-filter iscomputed in step 909 and an updated noise estimate is computed in step910. The parameters for the peak filter computed in step 907 are alsoused in step 912, which sets filter coefficients for the high-shelffilter stage 206.

Following step 910, step 911 evaluates one or more of the criteria of:

-   -   1) the noise level at one or more frequencies or frequency bins        or frequency bands is below a predefined threshold; or    -   2) the signal-to-noise level at one or more frequencies or        frequency bins or frequency bands is above a predefined        threshold or    -   3) all filter stages have been fitted since the process started        out in step 901.

In the negative event thereof (N) the process resumes at step 907 to fita further peak filter; whereas in the positive event thereof the processresumes to step 901 or completes itself until initiated again.

The coefficients computed in step 912 may be configured into the gain orfilter stages as they are computed while the process is running orimmediately before the process is complete or immediately thereafter.

Step 908 is performed in connection with step 907 to low-pass filter theamount of change that a peak filter is allowed to change from oneiteration to the next. Thereby non-linear distortion caused by changing(time varying) side-tone filtering may be kept at a low level, such asbelow an audible level.

1. A signal processor for a headset configured with a microphoneterminal for receiving a microphone signal, a loudspeaker terminal foroutputting a loudspeaker signal, and a far-end terminal forcommunicating an inbound signal and an outbound signal with a far-end;comprising: an outbound path from the microphone terminal to the far-endterminal; a side-tone path configured to generate a side-tone signalfrom the microphone signal via a side-tone filter, an inbound path fromthe far-end terminal to the loudspeaker terminal and comprising acombiner configured to combine the side-tone signal and the inboundsignal to generate the loudspeaker signal; and a side-tone filtercontroller that receives the microphone signal and computes a firstnoise estimate with a signal-to-noise level of the microphone signal atrespective frequency bands and based thereon controls the side-tonefilter to improve a signal-to-noise ratio of the side-tone signal;wherein the side-tone filter in the side-tone path comprises multiplefilter stages that are individually controlled based on the first noiseestimate; and wherein the multiple filter stages are configured from agroup consisting of: multiple peak filters and a high-shelf filter.
 2. Asignal processor according to claim 1, wherein the side-tone filtercontroller executes an iterative process of: determining parameters fora filter stage based on the first noise estimate; computing a frequencydomain gain estimate of the frequency domain gain transfer function fora filter stage; computing a second noise estimate by subtracting thefrequency domain gain estimate from the first noise estimate; updatingthe first noise estimate to be in accordance with the second noiseestimate; wherein the filter stages are controlled via coefficientscomputed from the parameters for a respective filter stage computed overiterations of the iterative process.
 3. A signal processor according toclaim 2, wherein determining parameters for a filter stage comprisesfitting a filter that has a largest possible gain-bandwidth productwhile optimizing the signal-to-noise ratio in an audio band of interest.4. A signal processor according to claim 2, wherein fitting a filtercomprises: over a range of frequencies, iteratively selecting afrequency; and determining a gain-bandwidth product of a band centred atthe frequency and determining a gain factor thereof based on thesignal-to-noise ratio and determining a bandwidth factor thereof as thebandwidth wherein the signal-to-noise ratio is not better thanapproximately half the signal-to-noise ratio at the frequency; when therange of frequencies has been iterated, selecting a favourablegain-bandwidth product, such as the largest gain-bandwidth product, andestimating parameters for a band-attenuate filter, such as apeak-filter, with a centre frequency and a gain to approximately fit thedetermined band.
 5. A signal processor according to claim 2, whereinfitting a filter comprises: fitting a high-shelf filter by selecting apredetermined shelf mid-point frequency and determining a shelf gain,such as an attenuation, from an average of the signal-to-noise ratio atfrequencies above the shelf mid-point frequency.
 6. A signal processoraccording to claim 1, wherein the signal processor: at a first time,computes a reference gain based on a minimum attenuation in the noiseestimate and scales the microphone signal based on the reference gain;and at a second time, following the first time, and iterativelyselecting a frequency.
 7. A signal processor according to claim 1,wherein the signal processor: at a second time, computes a frequencydomain gain estimate of a high-shelf filter, and then computes a secondnoise estimate by subtracting the frequency domain gain estimate fromthe first noise estimate; at a third time, following the second time,and iteratively selecting a frequency, wherein the filter is a stop-bandfilter or a peak-filter.
 8. A signal processor according to claim 1,wherein a cost function is defined for an optimization process; andwherein the cost function is a frequency domain estimate of thesignal-to-noise ratio which in a first iteration is computed from themicrophone signal and in subsequent iterations is computed from the costfunction and estimates in the frequency domain of selected filters; andwherein the optimization process iteratively selects a filter with anoptimization strategy to optimize the cost function and then updates thecost function.
 9. A signal processor according to claim 2, whereindetermining parameters for a filter stage by fitting a filter comprisesdampening the rate and/or distance the centre frequency and/or bandwidthis allowed to change over time.
 10. A signal processor according toclaim 1, comprising a voice activity detector which indicates presenceof speech; wherein a first amplitude or power spectrum for themicrophone signal is computed at a time when the voice activity detectoris detecting speech; wherein a second amplitude or power spectrum forthe microphone signal is computed at a time when the voice activitydetector is not detecting speech; and wherein the signal-to-noise ratiois computed from the first amplitude or power spectrum and the secondamplitude or power spectrum.
 11. A signal processing method forgenerating a side-tone in a headset, comprising: generating a side-tonesignal via a side-tone filter and from a microphone signal picked up viaa microphone of the headset, combining the side-tone signal and aninbound signal to the headset to generate a loudspeaker signal for aloudspeaker of the headset; computing a first noise estimate of themicrophone signal at respective frequency bands and based thereoncontrolling the side-tone filter to improve a signal-to-noise ratio ofthe side-tone signal; wherein the side-tone filter comprises multiplefilter stages that are individually controlled based on the first noiseestimate; and wherein the multiple filter stages are configured from agroup consisting of: multiple peak filters and a high-shelf filter. 12.A computer-readable medium carrying a program configured to perform themethod as claimed in claim 1, wherein the method is acomputer-implemented method run on a computer.
 13. A headset comprisinga voice microphone, an earpiece with a loudspeaker and a signalprocessor according to claim 1.